The invention relates to water based offset lithographic printing ink.
In an attempt to eliminate volatile organic compounds (VOCs) in the pressroom, water-based alternatives are being sought for ink formulations. Water-based printing inks for use in flexographic printing processes are known in the prior art. This type of printing process utilizes printing plates wherein the printing images stand up in relief, i.e. the areas to be printed are raised above the non-printing areas. Printing by the flexographic process requires relatively low pressure while sufficient pressure is applied to transfer the ink from the face of the image carrier to the surface of the substrate. Examples of useful water-based flexographic printing inks are disclosed in U.S. Pat. No. 4,173,554, U.S. Pat. No. 5,725,646 and The Printing Ink Manual, edited by R. H. Leach and R. J. Pierce, pages 571-576, 5th edition, (Blueprint, 1993).
Water-based inks for gravure printing are also well known. In the gravure process, the printing image is engraved into a cylinder in the form of cells which become filled with ink. Printing is achieved by passing the substrate between the gravure cylinder and impression roller under pressure. Examples of useful water-based gravure printing inks are disclosed in U.S. Pat. Nos. 4,954,556 and 5,098,478.
The offset lithographic printing process presents a unique challenge to ink formulators since such process utilizes a planographic printing plate, i.e. the image and non-image areas are in the same plane on the image carrier, and two fluids are concurrently utilized.
It is fairly simple to define an image area by raising it above the background as in the case of the flexographic printing plate or lowering it as in the case of the gravure printing plate; avoidance of ink adhering to the non-image area is not too difficult to achieve. However, when all areas are on the same level, techniques must be utilized to insure that ink adheres only to the image area, and not to the non-image area.
In conventional offset lithographic printing processes, the plate is damped before it is inked with an oil-based ink. Typically, the damping process utilizes a fountain solution such as those described in U.S. Pat. Nos. 3,877,372, 4,278,467 and 4,854,969. Water will form a film on the hydrophilic areas (i.e. the non-image areas) of the printing plate, but will contract into tiny droplets on the oleophilic areas (i.e. the image areas). When an inked roller containing the oil-based ink is passed over the damped plate, it will be unable to ink the areas covered by the water film (the non-image areas), but will emulsify the droplets on the water-repellant areas (the image areas) and these will ink up. Such process is called offset lithography because the inked image on the plate does not directly print onto the paper substrate, but is first xe2x80x9coffsetxe2x80x9d onto a rubber blanket, and transferred therefrom onto the paper substrate.
As mentioned above, conventional offset lithographic printing processes entails the use of oil-based inks and water-based fountain solutions. The ink/water balance is critical and is quite demanding of the pressman""s skills. This issue is one of the several disadvantages associated with such printing processes as compared to flexographic and gravure printing processes. Moreover, the oil-based inks and aqueous fountain solutions typically employed in conventional offset lithographic printing processes contain fairly high levels of undesirable volatile organic compounds (xe2x80x9cVOCsxe2x80x9d).
U.S. Pat. No. 3,356,030 discloses the use of a water-based printing ink in respect to a method of planographic printing utilizing a lithographic printing plate whose non-image areas are coated with a cured coating of a thermosetting silicone resin. However, the patented method also entails the use of a volatile hydrocarbon fountain solution which will coat the non-image areas and which is re-applied between successive printings. Of course, the use of a volatile hydrocarbon fountain solution undermines the principal purpose of the water-based ink compositions of the present invention, i.e. the avoidance of the use of volatile organic compounds (xe2x80x9cVOCsxe2x80x9d) during the printing process. Indeed, the water-based ink compositions of the present invention may be used for offset lithographic printing processes without any fountain solution whatsoever.
In the 1980s, a resurgence of interest occurred in respect to xe2x80x9cwaterlessxe2x80x9d lithographic printing processes. Both positive and negative waterless planographic printing plates are commercially available from Toray Industries of Japan. The image area of a waterless planographic plate is a photopolymer similar to that employed for the image area of a conventional plate. However, the non-image area is coated with a polymer such as a silicone which is ink repellant. Further information about waterless printing plates and processes may be found in U.S. Pat. Nos. 5,370,906 and 5,417,749.
The waterless printing process solved two issues: VOCs emanating from the fountain solutions and control of the ink/water balance by the pressman. However, the difference in surface energy between the image and non-image areas of the conventional offset lithographic printing plate is typically 40 dynes/cm is dramatically reduced to 20 dynes/cm in the case of the waterless printing plate. Therefore the latitude between scumming and poor print density is considerably narrowed and the issue of VOCs (emanating from the oil-based ink) still remains in respect to waterless printing.
German Offenlegungsschrift DE 41 19 348 A1 pertains to a moistureless offset printing method and a water-based printing ink. The ink described therein is one which will adhere to hydrophilic materials, but not to hydrophobic materials, and contains a dye, water, 5-50% water-soluble modified rosin polymer and a hygroscopic liquid, preferably a multihydric alcohol.
U.S. Pat. No. 5,725,646, which is incorporated herein by reference, describes a way of stabilizing a waterbased offset ink composition without drying up on a conventional multi roller ink train. Such composition eliminated the principal disadvantages of conventional offset lithographic printing inks, viz. high levels of VOCs emanating from the oil-based ink and the aqueous fountain solution and the difficulty in controlling the ink/water balance, while preserving the principal advantage of the conventional lithographic printing process, i.e. high surface energy differential between the image and non-image areas of the printing plate. It consists of using a rewetting agent, namely hydroxy ethyl ethylene urea. Since this rewetting agent does not dry, it remains in the final film making it susceptible to poor water resistance. Use of conventional rewetting agents, such as glycols and glycol ethers not only imparts poor water resistance to the final film, but also increases the tack of the formulations, thereby limiting its use on high speed presses.
Thus, there is a need in the art for a waterbased offset ink composition that is stable on conventional ink trains and exhibits good water resistance to the final film without increasing the tack of the formulation, thereby allowing for its use on high speed presses. A solution has been achieved by means of the present invention which comprises a water-based printing ink that is to be used in offset lithographic newspaper printing processes without the need for any accompanying fountain solutions.
Currently, all lithographic offset newspaper printing is done with inks which contain mineral oil or soy bean oils. These inks are also used in conjunction with a fountain solution which typically contains a desensitizer, a salt, and glycol. In order to prevent the evaporation of water from the printing rollers it was desirable to have constant humidity. This provided advantages over existing compositions, mainly zero VOC., water washability, and fast drying.
The prior art discloses a way of stabilizing a water based offset ink composition without drying up on a conventional multi roller ink trains. Such compositions consist of using a rewetting agent, namely hydroxyethyl ethylene urea. Since those rewetting agents do not dry, it remains in the final film, thereby making it susceptible to poor water resistance. Use of these conventional rewetting agents such as glycols and glycol ethers not only impart poor water resistance to the final film, but also increase the tack of the formulations, thereby limiting its use on high speed presses.
It has now been discovered that by reacting polyglycols, e.g., PEG 200, PEG 400, PEG 600, with linseed oil, the slow drying and poor water resistance problems associated with hydroxyethyl ethylene urea are overcome, while still maintaining water solubility of the compositions.
A water based offset lithographic news ink composition according to the present invention comprises: (a) water;(b) a modified rosin polymer comprised of: (i) resin soluble in water regardless of the pH of the water,(ii) resin rosin salts soluble in water at a pH ranging from about 7.5 to about 10, and (iii) aqueous emulsions resins; (c) a modified linseed oil; and (d) pigment.
The printing plates for use with the newspaper printing ink of the present invention should be such that the image areas thereof are hydrophilic in nature, while the non-image areas are hydrophobic in nature. An example of a suitable printing plate is the xe2x80x9cwaterlessxe2x80x9d Toray type discussed above. However, the image area of the plate need not contain a photopolymer. The image area of the plate may comprise, e.g. a grained aluminum surface which has no coating thereon, but is hydrophilic in nature. The non-image area of the plate must, of course, be hydrophobic in nature. However, the non-image area may be covered with any type of hydrophobic material, provided that such hydrophobic material adheres to the non-images area of the plate during the printing process.
The modified rosin polymer according to the present invention may be a maleic modified rosin esterified with pentaerithrytol. Examples of suitable modified rosin polymer which are soluble in the water phase of the ink regardless of the pH of the water phase include: carboxymethyl-cellulose, hydroxyethyl cellulose, hydroxypropyl-cellulose, hydroxybutylmethylcellulose, poly(C1-C4)alkylene oxides, polyethyleneimine, polyvinyl alcohol, polyvinyl acetate, polyvinylpyrollidone, polyvinyl-oxazolidone and polyacrylamide polymers.
Preferably, the modified rosin polymer present in the ink are only those which are soluble in the water at pH ranging from about 7.5 to about 10. Suitable examples of such modified rosin polymers include methacrylic resins; styrene-acrylic resins; rosin salts; and polystyrene-sulfonic acid and their salts. Ammonia or an organic amine such as monoethanolamine or N,N-diethanolamine may be added to the water phase in order to adjust the pH to the preferred value (a mineral acid or an organic acid such as acetic acid may be used to adjust the pH to a value in the range of about 2.5 to about 6.5).
Suitable examples of the modified rosin polymers comprising aqueous emulsions include acrylic or vinyl emulsion polymers prepared from monomers selected from the group consisting of acrylic acid esters, methacrylic acid esters, acrylic acid esters of polyhydric alcohols, methyl methacrylate, styrene, vinyl styrene and vinyl acetate.
The offset lithographic ink formula of the present invention employs a modified linseed oil. The modified linseed oil according to the present invention comprises the reaction product of a polyglycol and linseed oil. The modified linseed oil according to the present invention may also include lithium ricinoleate, phthalic anhydride and triphenyl phosphite.
The pigment may be any of those which are suitable for formulating offset lithographic printing inks such as CI Pigment Yellows 1, 3, 4, 5, 12, 13, 14, 17, 55, 65, 73, 83, 97 and 98; CI Pigment Oranges 13, 16 and 46; CI Pigment Reds 2, 3, 4, 10, 12, 48, 48:1, 48:2, 53, 57:2, 81, 104, 146, 170 and 176; CI Pigment Greens 2, 7 and 36; CI Pigment Blues 1, 15:1, 15:2, 15:3, 15:6, 16, 29, 56 and 61; CI Pigment Violets 3, 23 and 37; CI Pigment Blacks 6 and 7; and CI Pigment Whites 6, 7, 18 and 26.
In the water-based offset lithographic news ink of the present invention the water is present in amounts of 25 to 60 wt. %; and more preferably 35 to 50 wt. %. It is also preferred that the modified rosin polymers be present in amounts of 10 to 70 wt. %; and more preferably 30 to 60 wt. %; and most preferably the modified rosin polymers is a composite having up to 5 wt. % of a resin binder soluble in water regardless of the pH of the water; 10 to 70 wt. % of a resin binder soluble in water at a pH ranging from 7.5 to 10; and up to 20 wt. % of an aqueous emulsion resin binder. The modified linseed oil is present in amounts of 25 to 60 wt. %, and more preferably 35 to 55 wt. %. Finally, the pigment is preferably present in amounts of 2 to 30 wt. % and the soybean oil based modified resin is present in amounts of 15 to 35 wt %.
If desired, the usual adjuvants such as waxes, anti-foam agents, biocides, surfactants, corrosion inhibitors, etc. may be incorporated in the inks of the present invention. In another embodiment of the water-based offset lithographic printing ink of the present invention, a non-ionic surfactant is employed in the amount of up to 5 wt. %. Suitable examples of the surfactant include acetylenic glycols, ethoxylated glycols and sorbitan esters.